This invention relates generally to the field of motion picture projection, and more specifically to a system for correcting the position of film in a projector film gate relative to the focal plane of the projector lens.
General descriptions of motion picture projectors can be found in Motion-Picture Projectors and Theater Presentation Manual, Don V Kloepfel, Editor, Society of Motion Picture and Television Engineers, 1969, Library of Congress number 78-95387. The general construction of a conventional motion-picture projector is schematically illustrated in FIG. 7. Film projectors employ various types of film looping between feed and take-up reels. Traditional film projection systems have a film-transport wheel, called an intermittent sprocket, which pulls the film through a film gate one image frame at a time. Film loops in conventional systems compensate for rapid changes of un-reeled length of film above and below a film gate as a result of timed stops of the film at the film gates between evenly synchronized feed and take-up of the film by the respective reels. While stationed in the film gate, light generated in a lamp housing is directed through the film image frame, and through the projector""s projection lens assembly where it is focused for projection on a screen.
Motion picture films comprise transparent film supports with imaging layer(s) thereon which comprise the film images in the form of varying silver metal (in the case of black and white films) or dye (in the case of color films) densities in individual image frames. Motion picture projectors create a projected image on a screen by directing intense light through the motion picture film image frames while the frames are positioned in a projector film gate and imaging it to a screen. Because of the intense light, heat is created in the film when it is in the film gate. This heat can cause the film to buckle, flutter and bend. The heat is primarily generated in the film imaging layer(s) as it absorbs light in the dark areas of the image. This heat causes an increase in temperature that makes the film expand and move away from the focal plane of the projector""s projection lens assembly. The result is a less sharp projected image, because the motion of the film can move it outside of the depth of field of the projection lens.
Willy Borberg in a paper in the SMPTE Journal in October 1957 as well as an earlier paper in 1952 in the SMPTE Journal indicates that film flutter or motion of the film due to heating can cause film to move out of focus. His initial solution was to blow air on the film to keep it cool and to blow air to generate pressure to keep the film in the plane of focus. Additional prior art which discloses the use of blown air to cool or try to prevent film flutter includes U.S. Pat. No. 5,875,020. U.S. Pat. No. 5,341,182 discloses the use of blown air to advance film through a projection sequence, and the use of vacuum pressure to secure the film to a gate. U.S. Pat. No. 4,522,476 discloses the use of pressure pads which may be moved into and out of engagement with a film passing through the film gate of a projector. Although the use of blowing air has been used in a number of theatres (with or without additional film alignment tactics), there has been no attempt to compensate for the different amounts of heat absorbed by each image frame in a motion picture film. Nor has there been any attempt to compensate for the fact that the heat may be distributed non-uniformly across the image frame. In addition, the prior art has not provided a system to compensate for the fact that different films have different buckle characteristics due to base, age and warpage. Accordingly, it would be desirable to provide a system which would allow for active compensation of the position of the film in the film gate of a motion picture projector to selectively compensate for film flutter as a motion picture film is being projected.
In accordance with one embodiment of the invention, a system for compensating for film flutter in a motion picture film projector during projection of a motion picture film comprising multiple film image frames is described, the system comprising sensing the position of film image frames in a film gate of the projector relative to the focal plane of the projection lens assembly of the projector as the film image frames are being projected, and correcting the position of the film in the film gate of the projector or the position of the focal plane of the projection lens assembly to reduce the distance between the film image and the focal plane of the projection lens assembly as the film image frames are being projected, wherein the correction of the position of an individual film image frame in the film gate or that of the position of the focal plane of the projection lens assembly during projection of the individual frame image is based on the actual sensed position of the individual film image frame in the film gate, or on the position of a previously projected film image frame which was sensed as it was being projected.
In accordance with specific embodiments of the invention, light absorption characteristics of the film may be measured and used to predict the flutter or buckle of the film. The actual position of the film may then be sensed and the prediction algorithm modified so that the prediction is more accurate for the next frame of the motion picture image. This invention system actively senses the position of motion picture film in the film gate and adjusts the position of the film or focal plane of the projection lens assembly of the projector to bring the film image back into the focal plane of the projection lens during projection of the film. The film may be flat in the focus plane or may be curved depending on factors such as the projector and lens design.